Thin slabs of material are often inspected to determine thickness using known methods of observation and analysis of Fabry Perot interference fringes. In the case of a simple single slab of material, these known methods of inspection are based on the observation of interference fringes in an etalon formed by the parallel interfaces of the sample.
Some of the commonly measured slabs of anisotropic materials such as sapphire are bi-refringent in nature and used as wafer-carriers. Birefringence of the substrate implies that speed with which light propagates through such slab depends on polarization state of the light. Sapphire-plates are commonly used as wafer carriers for GaAs (Gallium Arsenide) based wafers. Accordingly, an accurate determination of thickness of the sapphire plates may be required while measuring thickness of GaAs patterned wafers residing on the sapphire-carrier.
However, employment of conventional methods of determining the thickness of the slab of material may be substantially inaccurate with respect to the aforesaid anisotropic materials at least at least due to birefringence. Reasons for such inaccuracy in some instances may be based on the thickness of the slab of anisotropic material being greater than about 50 m, measurement noise resulting from Schott noise, thermal noise, or the presence of stray light, or some combination thereof. Overall, the conventional methods for inspecting slab of material may not only have limited spectral-resolution, but may also not be effective when the slab of material to be measured exhibits birefringence and has the thickness as greater than 50 μm.
The subject matter claimed in this disclosure is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology area where some embodiments described in this disclosure may be practiced.